As in other imaging modalities; but, particularly in MRI, scientists are continually searching for methods and apparatus which can shorten the imaging time. In MRI the imaging time is comparatively long. One method that has been used in the past for decreasing the imaging time is known as the "driven equilibrium" pulse technique.
In driven equilibrium techniques used in conjunction with spin echo imaging sequences, the spins in the XY plane are "driven" back to alignment with the Z axis to shorten the time for the spins to reach equilibrium. For prior art explanations of the driven equilibrium techniques see an article entitled "Driven-Equilibrium Radio Frequency Pulses in NMR Imaging" by C. M. J. van Uijen et al which appeared in the Journal of Magnetic Resonance in Medicine, Volume 1 at pp 502-507 (1984) and U.S. Pat. No. 4,532,474. As noted in the article the driven equilibrium pulse techniques extend the possibility of manipulating image contrasts in pulse sequences with a high repetition rate. The data acquisition time is shortened considerably. What is particularly interesting and important is that the intensity of tissue with large T1's and T2's can be significantly enhanced. This is particularly important in imaging body sections such as cerebral spinal fluid (CSF) sections.
In the early days of NMR imaging, the imaging time was lengthened by the fact that an almost complete return of nuclear magnetizations to equilibrium preceded each excitation. The wait for equilibrium was done to prevent signal loss in imaging techniques with inherent low sensitivity.
In the pulse sequence proposed by C. M. J. Van Uijen et al for spin echo imaging with driven equilibrium, the phase of the spins prior to Z axis restoration is independent of magnetic field inhomogeneities. It is imperative for the success of the sequence, that:
(1) at a given point all the spins in the selected slice are precisely in phase; and
(2) the 90 degree Z restoring pulse is delivered along an axis which is exactly perpendicular to the direction along which the transverse magnetization is focused.
There are problems in achieving these criteria. The requirement for precise refocusing are extremely stringent; therefore, even small deviations caused by the eddy currents generated by the gradients are enough to seriously degrade the amount of magnetization restored to the Z axis.
Even when the spins are precisely refocused, the phase of the refocused spin is not predictable. Also due to the eddy currents generated responsive to the phase encoding gradient pulses, the phase of the last echo varies from cycle to cycle. Finally, the dependence of correct image contrast on perfectly rectangular sliced profiles is much stronger than with normal spin echo sequences. This results from the high sensitivity of the Z restoration to the slice profile.
Due to these problems, driven equilibrium techniques have not been generally utilized. Where they are utilized, they have not been reliable. Accordingly, scientists in the field are still endeavoring to find means and methods for using driven equilibrium to decrease the repetition time in spin echo imaging sequences.